1. Field of the Invention
The present invention pertains to means for controlling the coolant temperature in a motor vehicle, and more particularly, it pertains to a system that senses the temperature of the coolant supplied to the radiator of a motor vehicle and varies the speed of the radiator cooling fan in accordance with said temperature.
2. Description of the Prior Art
Motor vehicles use rotating fans to move air through the fins of a radiator in order to cool the liquid therein which is used to maintain the motor temperature below a predetermined value. Power to rotate the fan is usually coupled from the vehicle drive motor, by means of a belt and a pair of pulleys, so that the fan speed is proportional to the motor speed.
Modern vehicle motors are designed to operate most efficiently between a predetermined low value of temperature and a predetermined high value of temperature, so it is desirable that the cooling fan be disconnected when the temperature of the cooling liquid is below the predetermined low value. Also, the operation of the cooling fan requires a significant amount of horsepower, so it is desirable that the fan be turned off when it is not needed.
To increase efficiency, some present day motor vehicles utilize a clutch between a drive pulley and the cooling fan so that the fan will be disconnected and will not provide cooling until the motor temperature reaches a predetermined value. A temperature sensitive element, such as a wax pellet, may be used to activate and deactivate the clutch and thereby couple and decouple the fan to the drive motor. In these prior art vehicles the fan is usually completely decoupled from the drive motor when the motor temperature is below a predetermined value. When the motor temperature reaches said predetermined value the fan is directly connected to the drive motor so that the fan rotates at a speed proportional to that of the drive motor while the temperature remains above this predetermined value. During colder weather the fan may cause the radiator temperature to drop rather rapidly so that the fan is continually being turned on and then turned off thereby keeping the motor coolant temperature within a narrow temperature range.
Some of the prior art fan speed control systems utilize the temperature sensitive element to actually modulate the fan speed by controlling a variable drive coupling to the fan. Such prior art fan speed control systems provide temperature responsive speed control over a relatively narrow temperature range, however, due to the inherent limitations of the mechanical control element. For example, a change of 10 degrees of coolant temperature may cause the fan to go from off to full speed. These prior art controls also exhibit quite a large hysteresis band. That is to say, the fan may turn on at a given temperature and turn off at a temperature several degrees below the turn-on temperature. Furthermore, these prior art cooling fan control systems do not have any means for readily adjusting the range of temperatures over which they may operate, and they are non-linear and generally erratic in operation.